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Chapter 3:The Enhanced E-R Model

Modern Database Management10th Edition

Jeffrey A. Hoffer, V. Ramesh, Heikki Topi

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Objectives Define terms Understand use of supertype/subtype

relationships Understand use of specialization and

generalization techniques Specify completeness and disjointness constraints Develop supertype/subtype hierarchies for

realistic business situations Develop entity clusters Explain universal (packaged) data model Describe special features of data modeling

project using packaged data model

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Supertypes and Subtypes Enhanced ER model: extends original

ER model with new modeling constructs Subtype: A subgrouping of the entities in an

entity type that has attributes distinct from those in other subgroupings

Supertype: A generic entity type that has a relationship with one or more subtypes

Attribute Inheritance: Subtype entities inherit values of all

attributes of the supertype An instance of a subtype is also an instance

of the supertype

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Figure 3-1 Basic notation for supertype/subtype notation

a) EER notation

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Different modeling tools may have different notation for the same modeling constructs

b) Microsoft

Visio Notation

Figure 3-1 Basic notation for supertype/subtype notation (cont.)

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Figure 3-2 Employee supertype with three subtypes

All employee subtypes will have employee number, name, address, and date hired

Each employee subtype will also have its own attributes

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Relationships and Subtypes

Relationships at the supertype level indicate that all subtypes will participate in the relationship

The instances of a subtype may participate in a relationship unique to that subtype. In this situation, the relationship is shown at the subtype level

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Figure 3-3 Supertype/subtype relationships in a hospital

Both outpatients and resident patients are cared for by a responsible physician

Only resident patients are assigned to a bed

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Generalization and Specialization

Generalization: The process of defining a more general entity type from a set of more specialized entity types. BOTTOM-UP

Specialization: The process of defining one or more subtypes of the supertype and forming supertype/subtype relationships. TOP-DOWN

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Figure 3-4 Example of generalization

a) Three entity types: CAR, TRUCK, and MOTORCYCLE

All these types of vehicles have common attributes

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Figure 3-4 Example of generalization (cont.)

So we put the shared attributes in a supertype

Note: no subtype for motorcycle, since it has no unique attributes

b) Generalization to VEHICLE supertype

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Figure 3-5 Example of specialization

a) Entity type PART

Only applies to manufactured parts

Applies only to purchased parts

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b) Specialization to MANUFACTURED PART and PURCHASED PART

Note: multivalued attribute was replaced by an associative entity relationship to another entity

Created 2 subtypes

Figure 3-5 Example of specialization (cont.)

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Constraints in Supertype/ Completeness Constraint

Completeness Constraints: Whether an instance of a supertype must also be a member of at least one subtype Total Specialization Rule: Yes (double

line) Partial Specialization Rule: No (single

line)

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Figure 3-6 Examples of completeness constraints

a) Total specialization rule

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b) Partial specialization rule

Figure 3-6 Examples of completeness constraints (cont.)

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Constraints in Supertype/ Disjointness constraint

Disjointness Constraints: Whether an instance of a supertype may simultaneously be a member of two (or more) subtypes Disjoint Rule: An instance of the

supertype can be only ONE of the subtypes

Overlap Rule: An instance of the supertype could be more than one of the subtypes

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a) Disjoint rule

Figure 3-7 Examples of disjointness constraints

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b) Overlap ruleFigure 3-7 Examples of disjointness constraints (cont.)

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Constraints in Supertype/ Subtype Discriminators

Subtype Discriminator: An attribute of the supertype whose values determine the target subtype(s) Disjoint – a simple attribute with alternative

values to indicate the possible subtypes Overlapping – a composite attribute whose

subparts pertain to different subtypes. Each subpart contains a Boolean value to indicate whether or not the instance belongs to the associated subtype

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Figure 3-8 Introducing a subtype discriminator (disjoint rule)

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Figure 3-9 Subtype discriminator (overlap rule)

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Figure 3-10 Example of supertype/subtype hierarchy

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Entity Clusters

EER diagrams are difficult to read when there are too many entities and relationships

Solution: Group entities and relationships into entity clusters

Entity cluster: Set of one or more entity types and associated relationships grouped into a single abstract entity type

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Figure 3-13a Possible entity clusters for Pine Valley Furniture in Microsoft Visio

Related groups of entities could become clusters

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Figure 3-13b EER diagram of PVF entity clusters

More readable, isn’t it?

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Figure 3-14 Manufacturing entity cluster

Detail for a single cluster

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Packaged Data Models

Predefined data models Could be universal or industry-

specific Universal data model = a generic or

template data model that can be reused as a starting point for a data modeling project (also called a “pattern”)

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Advantages of Packaged Data Models

Use proven model components Save time and cost Less likelihood of data model errors Easier to evolve and modify over time Aid in requirements determination Easier to read Supertype/subtype hierarchies promote reuse Many-to-many relationships enhance model

flexibility Vendor-supplied data model fosters integration

with vendor’s applications Universal models support inter-organizational

systems

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Packaged data models are generic models that can be customized for a particular organization’s business rules

Figure 3-15 PARTY, PARTY ROLE, and ROLE TYPE in a universal data model

(a) Basic PARTY universal data model

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Figure 3-15 PARTY, PARTY ROLE, and ROLE TYPE in a universal data model

(b) PARTY supertype/subtype hierarchy

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